Selection of fit

When selecting a fit, the factors discussed in this section should be considered, together with the general guidelines provided.

1. Conditions of rotation

Conditions of rotation refer to the bearing ring being considered in relation to the direction of the load. Essentially, there are three different conditions: "rotating load", "stationary load" and "direction of load indeterminate".

"Rotating load" pertains if the bearing ring rotates and the load is stationary, or if the ring is stationary and the load rotates so that all points on the raceway are subjected to load in the course of one revolution. Heavy loads which do not rotate but oscillate, for example, those acting on connecting rod bearings, are generally considered as rotating loads.

A bearing ring subjected to a rotating load will turn (creep or wander), on its seat if mounted with a clearance fit, and wear (fretting corrosion) of the contact surfaces will result. To prevent this, interference fits must be used. The degree of interference needed is dictated by the operating conditions (see points 2 and 4 below).

"Stationary load" pertains if the bearing ring is stationary and the load is also stationary, or if the ring and the load rotate at the same speed, so that the load is always directed towards the same position on the raceway. Under these conditions, a bearing ring will normally not turn on its seat. Therefore, the ring need not necessarily have an interference fit unless this is required for other reasons.

"Direction of load indeterminate" represents variable external loads, shock loads, vibrations and unbalance loads in high-speed machines. These give rise to changes in the direction of load, which cannot be accurately described. When the direction of load is indeterminate and particularly where heavy loads are involved, it is desirable that both rings have an interference fit. For the inner ring the recommended fit for a rotating load is normally used. However, when the outer ring must be free to move axially in the housing, and the load is not heavy, a somewhat looser fit than that recommended for a rotating load may be used.

2. Magnitude of the load

The interference fit of a bearing inner ring on its seat will be loosened with increasing load as the ring will deform. Under the influence of rotating load the ring may begin to creep. The degree of interference must therefore be related to the magnitude of the load; the heavier the load, particularly if it is a shock load, the greater the interference fit required (fig 1). Shock loads and vibration also need to be considered.

3. Bearing internal clearance

An interference fit of a bearing on a shaft or in a housing means that the ring is elastically deformed (expanded or compressed) and that the bearing internal clearance is reduced. A certain minimum clearance should remain, however, see section "Bearing internal clearance". The initial clearance and permissible reduction depend on the type and size of the bearing. The reduction in clearance due to the interference fit can be so large that bearings with an initial clearance, which is greater than Normal, have to be used in order to prevent the bearing from becoming preloaded (fig 2).

4. Temperature conditions

In service, bearing rings normally reach a temperature that is higher than that of the components to which they are fitted. This can result in an easing of the fit of the inner ring on its seat, while outer ring expansion may prevent the desired axial displacement of the ring in its housing. Fast starting up or seal friction might also lead to easing of the inner ring fit.

Temperature differentials and the direction of heat flow in the bearing arrangement must therefore be carefully considered.

5. Running accuracy requirements

To reduce resilience and vibration, clearance fits should generally not be used for bearings where high demands are placed on running accuracy. Bearing seats on the shaft and in the housing should be made to narrow dimensional tolerances, corresponding at least to grade 5 for the shaft and at least to grade 6 for the housing. Tight tolerances should also be applied to the cylindricity (table).

6. Design and material of shaft and housing

The fit of a bearing ring on its seat must not lead to uneven distortion of the ring (out-of-round). This can be caused, for example, by discontinuities in the seat surface. Split housings are therefore not generally suitable where outer rings are to have a heavy interference fit and the selected tolerance should not give a tighter fit than that obtained with tolerance group H (or at the most K). To provide adequate support for bearing rings mounted in thin-walled housings, light alloy housings or on hollow shafts, heavier interference fits than those normally recommended for thick-walled steel or cast iron housings or for solid shafts should be used, see section "Fits for hollow shafts". Also, sometimes lighter interference fits may be required for certain shaft materials.

7. Ease of mounting and dismounting

Bearings with clearance fits are usually easier to mount or dismount than those with interference fits. Where operating conditions necessitate interference fits and it is essential that mounting and dismounting can be done easily, separable bearings, or bearings with a tapered bore may be used. Bearings with a tapered bore can be mounted either directly on a tapered shaft seat or via adapter or withdrawal sleeves on smooth or stepped cylindrical shafts (fig 4).

8. Displacement of the non-locating bearing

If bearings that cannot accommodate axial displacement within the bearing are used in the non-locating position, it is imperative that one of the bearing rings is free to move axially at all times. Adopting a clearance fit for the ring that carries a stationary load will provide this (fig 5). When the outer ring is under stationary load so that axial displacement is accommodated or takes place in the housing seat, a hardened intermediate bushing or sleeve is often fitted to the housing seat, for example, where light alloy housings are employed. In this way a "hammering out" of the housing seat because of the lower material hardness is avoided; it would otherwise result in the axial displacement being restricted or even prohibited over time.

If cylindrical roller bearings having one ring without flanges, needle roller bearings or CARB toroidal roller bearings are used, both bearing rings may be mounted with an interference fit because axial displacement will take place within the bearing.